Friction member and a method for its surface treatment

ABSTRACT

A method of surface treatment of friction members includes providing a friction member made of PMMC material. A transfer layer is formed on the active surface of the friction member of removing the top layer of the matrix material to expose a surface with the embedded reinforcing particles.

BACKGROUND OF THE INVENTION

The present invention relates to a method for surface treatment offriction members like brake discs, drums, clutch parts and, moreparticularly, to treatment of PMMC based members and friction membersprovided thereby.

Conventional brake discs are presently made of ferrous alloys/cast ironhaving satisfactory performance and remaining operative even atsubstantially elevated temperatures up to and above 700° C.

However, the present tendency in the automotive industry to reduce thetotal weight of vehicles challenges new lighter materials to alsopenetrate this particular segment of vehicle construction. Furthermore,improved corrosion resistance, as well as wear resistance, whichincrease the lifetime of the friction members up to the expected lifeperiod of the vehicles, are traits when looking for replacement of thepresent ferrous materials.

Consequently, several patent applications have been filed world-widerecently disclosing use of PMMC (Particle Metal Matrix Composite, suchas an Al-alloy matrix reinforced by ceramic particles) based componentsused for different actual applications in vehicles. Shortcomings incommon for all these applications based on PMMC base material are asoftening phenomena at elevated temperatures, something which results inscoring and even plastic deformation of the members' surface, thusconsiderably limiting the maximum allowed operating temperatures of themembers. Therefore, as a remedy, it is instrumental to provide the basicPMMC-made friction members either with a special composite/-ceramiccoating layer (thermal spraying of ceramics), or with a transfer surfacelayer.

The provision of an alternative transfer layer requires the layer to beboth stable (adherent to the substrate and reliable) and homogeneous.Furthermore, fast formation of the layer having a sufficient thicknessis also important both from a manufacturing, cost and performance pointof view.

One feasible way to cope with the task of increasing the maximumoperating temperature is simply to increase the volume percentage ofreinforcing particles. Unfortunately, two major disadvantages connectedwith this “solution”, namely increased costs of the PMMC base materialand difficulties related to production/casting and especiallycutting/-machining of the surface, eliminate this as a possibility for acost efficient manufacturing method.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novel,fast and cost efficient method of manufacturing friction members thatavoid the above mentioned drawbacks and difficulties connected to theconventional methods and products. Another object of the presentinvention is to provide a fast developing and homogeneous transfer layerexhibiting more stable friction properties, especially at high operatingtemperatures. Still another object of the present invention is toprovide better protection for the base matrix material against scoring.Still another object of the present invention is to provide betterprotection for the base matrix material against scoring. These and otherobjects and features of the present invention are met by a method ofmanufacturing friction members as discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail in the following by way ofexamples of preferred embodiments of the manufacturing method and theresulting members referring to FIGS. 1-4, wherein:

FIG. 1 is a perspective view showing a typical disc brake system;

FIG. 2 is a schematic view illustrating, in principle, the novel surfacetopography of the friction member treated in accordance with the presentinvention;

FIG. 3 is a microscope image of an untreated surface; and

FIG. 4 is a microscope image showing the same surface of FIG. 3, butafter exposure to an etching agent as described in the following underExamples.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, FIG. 1 shows a disc brake system 1 in whichthe brake disc 11 is the rotating part which together with the frictionlinings 15 held in place by the caliper 13 creates the friction.Although a brake disc is shown in the drawings, the present invention isalso applicable to clutch plates.

The novel surface treatment according to the invention is applied to thefriction surfaces 12 of the brake disc.

FIG. 2 illustrates schematically a detailed view of the surface of themember (disc) 12 treated in accordance with the present invention.

Contrary to the present practice and trend to add a special surfacelayer (e.g., in the form of a composite or sprayed ceramic layer) thegist of the present novel treatment method lies in a selective partialremoval of the base matrix material from the active to be frictionalsurface(s) of the member.

FIG. 2 shows in a cross sectional view the (top) surface 2 of the brakedisc 11 in which the original top layer depicted as 23 is removed, mostadvantageously by chemical etching. This treatment results in a novelsurface topography including a surface with reinforcing (ceramic)particles 22 protruding from the matrix 21. Later, during the initialbreak-in activation of the brake system, the particles 22 become anintegrated part of the transfer layer created through initial wear andmaterial transfer from the lining (pad) material. The resultingincreased reinforcement of the transfer layer will provide betterprotection of the matrix alloy from temperature and shear forces.

Tests conducted on samples of PMMC discs surface treated in accordancewith this method confirm formation of a fast developing adherent andhomogenous transfer layer exhibiting substantially improved performancecharacteristics of the treated member.

Furthermore, tests have shown that etching increases the pad wear duringthe initial use of the disc during the creation of the transfer layer.The degree of etching should therefore be chosen to reach an acceptableinitial pad wear. After creation of the transfer layer the actual padwear falls to a lower level.

EXAMPLES

Samples of brake discs made of two different AlSiMg matrix alloysreinforced by SiC particles in an amount of 10 to 30 vol % having a sizein a range from 5-30 μhave been subjected to chemical etching applying asolution of NaOH in concentrations from 5-30% up to 20 minutes.Comparison to the reference samples based on the measurement of surfaceroughness, friction and performance at elevated temperatures showsimproved characteristics on all measured parameters.

A relatively short etching time proved to be adequate to remove asufficient amount of the aluminium matrix, allowing the SiC particles toprotrude from the surface of the brake disc as illustrated by theattached FIGS. 3 and 4 showing sample surfaces before and after theetching treatment according to the present invention, respectively.

The actually-applied disc brake samples were made of AlSiMg alloy added20 weight % of SiC particles. The surfaces 2 of the brake discs 11 wereexposed for a period of 2 minutes to 12 weight % water solution of NaOH.(Reference number 3 depicts an Al-foil material applied on the samplesas protection for the surfaces prior to microscopic evaluation of theachieved results).

As clearly illustrated in FIG. 4, an exposure time of 2 minutes wassufficient to provide an etched surface 2 with SiC particles 4protruding from the surface 2.

Generally, an etching time from 1-3 minutes and an application of 12.5weight % NaOH solution at room temperature is apparently sufficient toachieve an adequate degree of etching of the surface. Prolonged etching(in excess of 5 minutes) can result in loosening of SiC particles. Thetemperature and the control of the flow of the etching agent willdetermine the choice of optimal etching time.

The present invention is not limited to the above-described examples ofthe preferred mode of the surface treatment. Thus, other (similar)methods of surface treatment (such as electrochemical pickling orchemical etching by means of an appropriate acid) could be used withoutdeparting from the spirit and scope of the present invention. Inaddition, alkali-based etching agents other than the exemplified NaOH,such as KOH, could be used. Also, other types of PMMC material withother reinforcing particles like Al₂O₃ instead of the above describedSiC-reinforced Al-matrix are the most practical alternatives.

What is claimed is:
 1. A method of forming vehicle components, saidmethod comprising: integrally forming a friction member on either abrake disk, a clutch plate or a brake disk and a clutch plate, thefriction member being formed of a PMMC material including an Al-alloymatrix material and ceramic reinforcing particles embedded in the matrixmaterial; and forming a transfer layer on a friction surface of thefriction member, said forming of the transfer layer including removingthe top surface layer of the matrix material so as to expose a surfaceof the ceramic reinforcing particles to thereby increase a frictioncoefficient of the friction surface of the friction member, saidremoving of the top surface layer of the matrix material comprisingchemical etching of the PMMC material.
 2. The method of claim 1, whereinsaid chemical etching comprises applying NaOH in a concentration in arange of 5% to 30% by weight as the etching agent.
 3. The method ofclaim 1, wherein said chemical etching comprises applying an acidreagent as the etching agent.
 4. The method of claim 1, wherein saidchemical etching comprises applying KOH as the etching agent.
 5. Amethod of forming vehicle components, said method comprising: integrallyforming a friction member on either a brake disk, a clutch plate, or abrake disk and a clutch plate, the friction member being formed of aPMMC material including an Al-alloy matrix material and ceramicreinforcing particles embedded in the matrix material, and forming atransfer on a friction surface of the friction member, said forming ofthe transfer layer including removing the top surface layer of thematrix material so as to expose a surface of the ceramic reinforcingparticles to thereby increase a friction coefficient of the frictionsurface of the friction member, wherein said removing of the top surfacelayer of the matrix material comprises electrochemical pickling of thePMMC material.